1. Field of the Invention
Embodiments of the present invention generally relate to a method for estimating subsurface reservoir properties, such as porosity and saturation.
2. Description of the Related Art
For many years seismic exploration for oil and gas has been conducted by use of a source of seismic energy and the reception of the energy generated by the source by an array of seismic detectors. On land, the source of seismic energy may be a high explosive charge or another energy source having the capacity to deliver a series of impacts or mechanical vibrations to the earth's surface. Acoustic waves generated by these sources travel downwardly into the earth's subsurface and are reflected back from strata boundaries and reach the surface of the earth at varying intervals of time, depending on the distance traveled and the characteristics of the subsurface traversed. These returning waves are detected by the sensors, which function to transduce such acoustic waves into representative electrical signals. The detected signals are recorded for later processing using digital computers. Typically, an array of sensors is laid out along a line to form a series of detection locations. More recently, seismic surveys are conducted with sensors and sources laid out in generally rectangular grids covering an area of interest, rather than along a single line, to enable construction of three dimensional views of reflector positions over wide areas. Normally, signals from sensors located at varying distances from the source are added together during processing to produce “stacked” seismic traces. In marine seismic surveys, the source of seismic energy is typically air guns. Marine seismic surveys typically employ a plurality of sources and/or a plurality of streamer cables, in which seismic sensors are mounted, to gather three dimensional data.
Initially, seismic traces were used simply for ascertaining formation structure. Recently, however, exploration geophysicists have subsequently developed a plurality of time-series transformations of seismic traces to obtain a variety of characteristics that describe the traces, which are generally referred to as “attributes”. Attributes may be computed prestack or poststack. Poststack attributes include reflection intensity, instantaneous frequency, reflection heterogeneity, acoustic impedance, velocity, dip, depth and azimuth. Prestack attributes include moveout parameters such as amplitude-versus-offset (AVO), and interval and average velocities.
It has been observed that specific seismic attributes are related to specific subsurface properties. For example, reservoir porosity and the hydrocarbon saturation may be estimated from surface seismic data to predict the amount of oil or gas reserves in the subsurface reservoirs. Generally, the reservoir porosity is estimated while keeping the hydrocarbon saturation fixed, and the hydrocarbon saturation is estimated while keeping the reservoir porosity fixed. Such methodology, however, often leads to inaccurate estimates of the porosity and saturation.
Therefore, a need exists in the art for an improved method for estimating porosity and saturation in a subsurface reservoir.